Windmilling propellers behaviour during a dive

[F/JG300_Gruber]

Hi folks,

 

I noticed something tonight derping around in quick missions, and if someone could explain me why it behaves like that I would be grateful.

 

When taking a plane at high altitude (9000m and above) and setting them in a dive, once you reach a certain speed the engine RPM will start increasing despite the prop pitch being stuck at full coarse.

Now if you keep the same indicated airspeed during the whole dive, once you get below 4000m or so, the RPM will start to decrease even if the propeller pitch is not changing.

 

I've tested the MC202 and the 109F2 and both behave the same : At a stabilized IAS of 820 km/h, idle throttle and full coarse pitch, the engine will wind up above 2500-2600rpm (potentially killing the Macchi engine) above 6000m but will gradually slow down to normal values as the ground approaches.

 

In my basic knowledge, if you maintain the same airspeed (IAS) during the whole dive, the airflow pressure on the propeller blade should remain the same and thus the RPM should be constant during the whole dive right ? So why is it decreasing once you reach mid to low altitudes ? What am I missing here ?

Edited November 18, 2016 by F/JG300_Gruber

[-=PHX=-SuperEtendard]

Maybe the denser air at low altitudes slows down the propeller via drag or "weight" (aka resistance to be moved). The constant indicated air speed would mean the same force is applied to the propeller to turn (airflow pressure).. but when the propeller turns it "digs" a certain volume of air, at lower altitudes this same volume has more weight to move as the air is denser, presenting more resistance to the propeller and with this offseting the previous equilibrium point (propeller RPM) achieved at higher altitudes.

 

Don't if it's correct but it's just a thought.

[Guest deleted@50488]

Surely air density, which plays also a big role in aerodynamics.

 

You'll also be able to notice differences from Winter cold to Summer hot scenery.

[L3Pl4K]

 

 

Hi folks, I noticed something tonight derping around in quick missions, and if someone could explain me why it behaves like that I would be grateful. When taking a plane at high altitude (9000m and above) and setting them in a dive, once you reach a certain speed the engine RPM will start increasing despite the prop pitch being stuck at full coarse. Now if you keep the same indicated airspeed during the whole dive, once you get below 4000m or so, the RPM will start to decrease even if the propeller pitch is not changing. I've tested the MC202 and the 109F2 and both behave the same : At a stabilized IAS of 820 km/h, idle throttle and full coarse pitch, the engine will wind up above 2500-2600rpm (potentially killing the Macchi engine) above 6000m but will gradually slow down to normal values as the ground approaches. In my basic knowledge, if you maintain the same airspeed (IAS) during the whole dive, the airflow pressure on the propeller blade should remain the same and thus the RPM should be constant during the whole dive right ? So why is it decreasing once you reach mid to low altitudes ? What am I missing here ?

Same problem here:

http://www.deutscheluftwaffe.de/archiv/Dokumente/ABC/m/Messerschmitt/Me%20109/Hochgeschwindigkeits_Versuche_Me109.pdf

It is impossible to copy the test flights from page 8 and 9.
The ingame engine will overrev. The Engine in the real world will not.

[F/JG300_Gruber]

Ok, I didn't thought about air density acting in that direction, that may be an explanation.

 

For L3Pl4K, Han mentionned recently that they don't have official documentation about the german propeller blade characteristics for the 109 series, so they are modelled in game by approximations regarding performances or something like that. That would explain the difference between in game and real life behaviour.

[Holtzauge]

In my basic knowledge, if you maintain the same airspeed (IAS) during the whole dive, the airflow pressure on the propeller blade should remain the same and thus the RPM should be constant during the whole dive right ? So why is it decreasing once you reach mid to low altitudes ? What am I missing here ?

 

I think a good way to understand this is if you think in terms of velocity vectors: The explanation for the varying prop rpm even though you maintain a constant IAS is this I think: The angle of attack that the propeller perceives is the combination of the rotational and the forward motion air speeds in absolute speeds, i.e.TAS. Now if we start of at a certain IAS at a certain height and dive while maintaining the IAS constant, the forward TAS will decrease due to the increased air density. This results in a reduction of the driving angle of attack the propeller blade perceives (even if pitch is kept constant) which will consequently slow down the rotational component i.e. the rpm.

 

So contrary to indicating that something is off in the propeller FM I think this piece of behaviour in IL-2 actually shows that the BoX FM is quite advanced.

[-WILD-AlbinoHA5E]

I would also go with the IAS-TAS Problem. The Prop Revs in a Dive seem to be more of a Product of something inbetween the two, and as the difference decreases so will the RPM.

[72AG_terror]

It is not an IAS-TAS issue, it is bit simpler than that. IAS is basicaly the ram air pressure the Pitot tube 'feels' and if the ram pressure changes for the Pitot tube, it changes by the same amount for the whole plane including the propeller. If you fly a constant IAS, the ram pressure on the propeller is constant as well and so is the driving force rotating the propeller. On the other hand however as the prop's blade moves through the air it pulls some air with it and hence experiences a certain amount of drag as it stirs the air and as you decend this drag increases proportionaly to the air density (it is easier to stir, say, water than honey). It is obvious that since the driving force is constant while drag  (slowing down force) is increasing the prop will 'try' to slow down with the descent. If the prop was of fixed pitch we would see a constant RPM decrease while descending at constant IAS, our prop however can compensate by lowering its drag through decreasing blades' angle of incidence, but there is a limit to that and as the governor reaches prop's shallowest angle of incidence and is unable to reduce the prop's drag any further the RPM begins to drop.

Edited November 28, 2016 by 72AG_terror